534
DESIGNING THE PEGASUS
the fan or core of an engine conventionally situated in
the rear fuselage would merely create an unbalanced
moment tending to pitch the nose down. From the begin
ning it was obvious that the Pegasus would be a two-spool
engine in order to meet the performance requirement.
There was immediately the possibility of distributing the
lift thrust between the low-pressure and high-pressure
systems. This was convenient because it enabled the nozzle
size to be kept down (four small ducts compared with two
large ones).
The bypass ratio, about 1-35:1, was settled by the need
to ensure approximately equal thrust from the front (cold)
and rear (hot) nozzles. This meant keeping the products of
the mass flow and gas velocities comparable. Since the
velocity through the fan is lower than through the hot
section, its mass flow has to be greater, and hence the
deviation from what, at first sight, should be a 1:1 ratio.
The pressure ratio, 13:1, was settled by the need for maxi
mum range. A plot of range versus pressure ratio shows a
shallow curve with range initially increasing but eventu
ally levelling off and then falling. The reason for this
characteristic is that, while increasing pressure ratio makes
for better efficiency and decreasing s.f.c, the effect of the
extra weight needed to counter the greater internal engine
stresses eventually becomes predominant.
Upgrading of the engine means more than simply
increasing the hot-end thrust by running the turbines
hotter, since this would eventually unbalance the thrust
centre from the e.g. of the aircraft. De-coupling the fan
exhaust from the turbine efflux has the advantage that
changes or modifications to one spool can be made without
significant effects upon the other. Improvement pro
grammes can be directed independently at the front or
rear of the engine, subject to the need to keep the thrust
centre substantially unchanged. And power increases—
more than with any other class of aeroplane—are always
needed to keep pace with increases in weight and
demanded performance; in V/Stol there is hardly such a
thing as excess thrust. This approach to thrust improve
ment does mean, however, that drastic measures, e.g. the
introduction of a zero-stage compressor, cannot be made
without badly disturbing the thrust centre.
The Pegasus 6 was the first production engine and differs
FLIGHT International, 19 October 1972
This view of the Pegasus 6 shows the large-diameter titanium fan — the
casing is 48in across—and the snubber ring near the tips of the blades
to damp out vibration in these thin components. The three stages are
overhung from the low-pressure shaft. Below, a "Flight" study of a
two-seat Harrier (this one has an extra-long tailboom housing an anti-
spin parachute, for test purposes)
from the -5 chiefly in having an all-titanium fan. It also has
a water-injection system for thrust restoration in hot-and-
high conditions. The interim Pegasus 10 was, it seems, a
rather unexpected half-way step to the 11 necessitated by
fairly tight financing by the British Government. The goal
of the Pegasus 10 was the development of the turbine,
leaving the fan improvement as the object of the Pegasus
11. The first moved the thrust centre somewhat aft, the
second restored it to the original position.
The Mk 103, Pegasus 11 has a re-designed fan to
permit a greater mass flow, and a slightly increased pres
sure ratio. The spools rotate in opposite directions and
their resultant gyroscopic forces very largely cancel one